1. Field of the Invention
This invention relates generally to a system and method for recovering fuel cell stack voltage loss and, more particularly, to a system and method for recovering fuel cell stack voltage loss that includes providing liquid water at the stack electrode surface, operating the stack at a low cell potential and flowing water through the fuel cell stack.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte there between. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically, but not always, include finely divided catalytic particles, usually a highly active catalyst such as platinum (Pt) that is typically supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
A fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow fields are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow fields are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The membrane within a fuel cell needs to have sufficient water content so that the ionic resistance across the membrane is low enough to effectively conduct protons. Membrane humidification may come from the stack water by-product or external humidification. The flow of reactants through the flow channels of the stack has a drying effect on the cell membranes, most noticeably at an inlet of the reactant flow. However, the accumulation of water droplets within the flow channels could prevent reactants from flowing therethrough, and may cause the cell to fail because of low reactant gas flow, thus affecting stack stability. The accumulation of water in the reactant gas flow channels, as well as within the gas diffusion layer (GDL), is particularly troublesome at low stack output loads.
Wet stack operation, that is, operation with a high amount of humidification, is desirable for system humidification, performance and contaminant removal. However, there are various reasons to operate a fuel cell stack with a lower amount of humidification, also known as a dry operating condition. For example, wet stack operation can lead to fuel cell stability problems due to water build up, and could also cause anode starvation resulting in carbon corrosion. In addition, wet stack operation can be problematic in freeze conditions due to liquid water freezing at various locations in the fuel cell stack. Therefore, there is a need in the art for systems that have been optimized for non-wet operating conditions.
In a fuel cell system, there are a number of mechanisms that cause permanent loss of stack performance, such as loss of catalyst activity, catalyst support corrosion and pinhole formation in the cell membranes. However, there are other mechanisms that can cause stack voltage losses that are substantially reversible, such as the cell membranes drying out, catalyst oxide formation, and a build-up of contaminants on both the anode and cathode side of the stack. Therefore, there is a need in the art to remove the oxide formations and the build-up of contaminants, as well as to rehydrate the cell membranes, to recover losses in cell voltage in a fuel cell stack.
In order for a PEM fuel cell system to be commercially viable, there generally needs to be a limitation of the noble metal loading, i.e., platinum or platinum alloy catalyst, on the fuel cell electrodes to reduce the overall system cost. As a result, the total available electro-chemically active surface area of the catalyst may be limited or reduced, which renders the electrodes more susceptible to contamination. The source of the contamination can be from the anode and cathode reactant gas feed streams including humidification water, or generated within the fuel cells due to the degradation and decomposition of the membrane, stack sealants and/or bipolar plates. One particular type of contaminate includes anions, which are negatively charged, such as chlorine or sulfates, such as SO4. The anions tend to adsorb onto the platinum catalyst surface of the electrode during normal fuel cell operation when the cathode potential is typically over 650 mV, thus blocking the active site for oxygen reduction reaction, which leads to cell voltage loss. Moreover, if proton conductivity is also highly dependent on a contaminate free platinum surface, such as nano-structured thin film (NSTF) type electrodes, additional losses are caused by the reduced proton conductivity.
U.S. patent application Ser. No. 12/580,912, filed Oct. 16, 2009, titled Automated Procedure For Executing In-Situ Fuel Cell Stack Reconditioning, assigned to the assignee of this application and herein incorporated by reference, discloses a system and method for reconditioning a fuel cell stack that includes increasing the humidification level of the cathode side of the stack to hydrate the cell membranes and providing hydrogen to the anode side of the fuel cell stack at system shut-down, where the system monitors reconditioning event triggers, reconditioning thresholds and reconditioning system checks so that the reconditioning process can be provided during vehicle operation.
Generally, stack reconditioning includes running the fuel cell stack with high relative humidity to remove contaminates from the stack to recover from stack degradation. However, reconditioning is an abnormal operation and exposes the stack to wet operations that may cause reliability issues if liquid water ends up in anode flow-fields and low anode flow rates are not able to purge them out. Thus, reconditioning should be performed only when it is absolutely necessary. Previous stack reconditioning triggers included triggering the reconditioning by monitoring the number of vehicle trips or key cycles. If the number of trips exceeded a threshold, which is considered as a representation of time after which stack voltage has degraded, the reconditioning process is triggered. However, improvements in triggering the reconditioning process can be made so that the reconditioning is only performed when necessary to reduce the abnormal operation conditions.
It is also known in the art that some of the contaminants that form on the electrodes in a fuel cell stack can be removed from the electrode by oxidizing the contaminant. In order to oxidize the contaminants on the electrodes, it is necessary to raise the potential across the electrodes to a high enough voltage to provide that oxidation. However, the theoretical maximum potential of a hydrogen fuel cell stack within a typical fuel cell system on a vehicle is lower than the necessary voltage potential.